Cellular communication networks constantly evolve towards higher data rates, together with improved capacity and coverage. In the 3rd Generation Partnership Project (3GPP) standardization body technologies like Global system for Mobile Communication (GSM), High Speed Packet Access (HSPA) and Long Term Evolution (LTE) have been and are currently developed.
LTE is the latest technology to be standardised. It relies on an access technology based on OFDM (Orthogonal Frequency Division Multiplexing) for the downlink (DL) and Single Carrier FDMA (SC-FDMA) for the uplink (UL), see 3GPP TS 36.201, Evolved Universal Terrestrial Radio Access (E-UTRA); Long Term Evolution (LTE) physical layer; General description, version 800.
The resource allocation to user equipments (UEs), both in the downlink channel and the uplink channel, is performed adaptively employing fast scheduling and taking into account the instantaneous traffic pattern and radio propagation characteristics of each UE. Assignment of resources for the downlink and for the uplink is performed in a scheduler located in the eNodeB of the LTE system.
To enable an efficient scheduling based on instantaneous radio characteristics, the scheduler needs information about current channel conditions. In LTE the downlink scheduler uses Channel Quality Indicators (CQI) reports that are transmitted on the uplink channel from the UE to the scheduler and contains information about the downlink channel characteristics for this particular user in order to make proper allocation decisions.
In an LTE system, the CQIs can be of various types, wideband and frequency selective. The report-formats for an UE are configured by the Radio Resource Control (RRC) layer. A full description of all report-formats can be found in 3GPP TS 36.213 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures.
CQI reports contain information obtained by measuring the signal strength on downlink channel reference symbols and interference estimations. A CQI value indicates a transport format (coding and modulation) for parts of or the entire downlink channel that gives an acceptable error rate.
In the current LTE standard, CQI reports can be transmitted on two alternative channels. One is the Physical Uplink Control Channel (PUCCH), where a UE can be assigned a periodic resource. The assignment and revocation of these resources are configured by the RRC layer. Also, CQIs can be transmitted on the Physical Uplink Shared Channel (PUSCH). The CQIs transmitted on the Physical Uplink Shared Channel are event or timer based transmissions that are scheduled and transmitted time multiplexed together with regular uplink data. To indicate if the UE is to transmit a CQI report on the PUSCH, the uplink channel grant given by the scheduler sets a dedicated “CQI indicator” bit to 1, otherwise it is 0, see 3GPP TS 36.212 Evolved Universal Terrestrial Radio Access (E-UTRA); Multiplexing and channel coding and 3GPP TS 36.213 Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures.
If a PUSCH transmission coincides with a PUCCH CQI transmission the CQI will be transmitted on the PUSCH. In this case the CQI format configured for the PUCCH will be used unless the “CQI indicator” bit is set. If the “CQI indicator” bit is set it will override and the PUSCH CQI format is used.
The PUCCH in LTE is used not only for CQI reports but also for scheduling requests (SRs) and HARQ ACK/NACK feedback for downlink data. The PUCCH resource is located on resource blocks (RBs) along the bandwidth edges of the UL frequency band and frequency hopping is used to gain frequency diversity.
The number of resource blocks allocated for PUCCH can be configured to basically any appropriate value. On the PUCCH resource blocks, transmissions are multiplexed using a code division. The basic resource per pair of resource blocks is the number of orthogonal codes available for transmission within a cell.
On a general level, the LTE standard supports 18 orthogonal resources per pair of resource blocks. Further, an ACK/NACK and scheduling requests transmissions occupies one of these codes enabling 18 simultaneous scheduling requests or ACK/NACK transmissions while a CQI transmission requires three times the resource enabling at most 6 CQI transmissions per pair of resource blocks. These transmissions may also be transmitted together on the channel, in which case a CQI transmission can be traded against 3 scheduling requests or ACK/NACKs.
Another aspect of the CQI reporting is the support of periodic PUSCH allocation by means of a persistently scheduled resource. With a persistent scheduling the UE will be allocated a specified resource with a certain periodicity without the need for a specific grant for every allocation. Also in this persistent grant the CQI indicator can be set, giving a periodic CQI reporting on PUSCH.
As described above the CQI reports can be transmitted on PUCCH as well as PUSCH. The PUCCH resource is periodic and assigned by the radio resource control. It is revoked by radio resource control or if the UE falls out of synchronization, which is timer controlled at the UE or at cell reselection, e.g. at handover.
To enable efficient scheduling decisions within the LTE standard, the CQI reports must be up to date, implying that they must be transmitted rather frequently to be effective. A UE with a periodic CQI resource allocated on the PUCCH will use this on every occasion. The exception is if it is simultaneously, i.e. within the same Transmission Time Interval TTI, scheduled for a PUSCH transmission, in which case the PUCCH resource will be left unused.
One problem with the existing method of transmitting CQI reports is that the PUCCH has a low load requirement and therefore is an expensive resource in terms of radio resources. The fact that an allocated periodic CQI resource on the PUCCH will be used by UEs not involved in concurrent uplink transmission can imply that substantially less than the six maximum CQI users should be assigned per TTI and pair of resource blocks, in order to secure robustness on the interference sensitive PUCCH.
To frequently revoke and reallocate the PUCCH resources by means of radio resource control is not feasible on a fast time scale and the radio resource control messages are high priority signaling messages, which are expensive from a radio resource perspective. Other possibilities to solve the allocation problem using support from the current standard are to assign the CQI resources on the PUCCH very infrequently, or alternatively allocate more resource blocks for PUCCH usage. Both of these methods are inefficient and will degrade system performance.
Hence, there exist a need for a method and a system that is able to provide a more efficient use of resources, such as PUCCH resources in a cellular telecommunication system.